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As plants put on their early spring growth, they need nutrients during February and March. Start fertilization when you see the plants begin to grow, when buds begin to swell. The first flush of growth for trees, shrubs, and perennial plants use stored reserves of nutrients from the previous growing year. Replacing these reserves is a vital part of keeping plants healthy. The purpose of fertilizer is to provide necessary plant nutrients so that the plant can make its own food through photosynthesis. Choosing and using fertilizer properly will enhance plant growth and protect water quality.
Not all trees and shrubs will need fertilizing. Large established plants can frequently get along fine with a spring compost mulch, or other mulch, and no supplemental fertilizer. What are the signs of a plant that is growing well? If the plant develops good leaf color, puts on an average amount of new shoot growth and length per season, and generally appears healthy, fertilizer may not be needed.
It's easy to over fertilize. Gardeners often assume landscape plants need lots of 'plant food' and apply too much fertlizer too frequently. Be sure to follow label directions for quantity and do not add fertilizer to tree and shrub plantings after active spring growth has finished. This means eliminating fertilizer applications to landscape plants after June. If you apply fertilizer too late in the season, the plant may fail to go into normal fall and winter dormancy and can be harmed by early freezes.
A new landscape does need spring fertilization. If the garden has newly planted trees and shrubs, installed within the last 2 to 4 years, be sure to fertilize these to ensure best growth. Roots that are just getting established need extra nutrients in their second year. If you are planting now, do not put fertilizer into the planting hole below the roots. Instead, use 2-3 inches of mulch on top of the root ball. Fertilize the plant when the roots have had at least 6 months to get established, preferably in the following spring after planting.
Nitrogen is the one nutrient commonly needed for good spring growth in the Pacific Northwest. Since it is leached out of soil by winter rains it should be replaced. You can choose a basic uncomplicated fertilizer such as a 5-10-10 (5 parts available nitrogen, 10 parts available phosphorous, 10 parts available potassium). Slow release formulas provide nutrients released gradually and they are useful for landscape plants. Even a basic lawn fertilizer, without weed-killers, can serve as a fine all-purpose plant fertilizer. (For instance, a 21-7-14 lawn fertilizer will work well.) Be sure that there is no weed-killer in the mix, because the herbicides will damage or kill landscape plants.
In addition to recently installed plants, also fertilize any that are not putting on sufficient new growth, have shown smaller leaves than normal, or are "off color" and yellowed. These symptoms may indicate the need for nitrogen. Spread the fertilizer across the entire root zone.
You do need some diagnosis of what's happening with the plant. Be sure that the symptoms described above, such as yellowing leaves, aren't connected to root damage or root rot. The symptoms of lack of fertilizer are similar to the symptoms of other sorts of plant problems, particularly root rots. Before fertilizing, check the root structure of the plant.
If you suspect a root problem, dig gently around the base of the plant to look at the root structure. Roots that are slimy, brownish black, very short, or clearly decayed will not be able to take in fertilizer. Dumping nutrients on a plant with a failing root system won't improve the plant. It will waste the fertilizer, and may result in nutrients being washed away into surface water where they can contribute to pollution problems.
Fertilizing landscape plants isn't just a matter of buying a bag of something and applying it. Observe the health and stage of growth of the plant first. Avoid putting a high-nitrogen fertilizer on a slope, or putting out fertilizer during a heavy rain that may cause it to run off. Apply the right amount in the right place at the right time!
For more information on different types of fertilizer use in the home and garden, check the Dial-Extension tapes in the King County Washington State University WSU Extension office. Call 206-296-3425 then dial in the Tape#1523 "Fertilizing and Mowing Lawns." Other tapes are #1967 "Fertilizing Vegetable Gardens" and #1651 "Fertilizing Trees and Shrubs." Tape #1651 gives some specific suggestions on poundage of fertilizer per inch of stem or width of shrub, or call your local WSU Extension Office.
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